Recent work in Medical Informatics is exploring the development and the use of formal ontologies as a way of specifying content-specific agreements for the sharing and reuse of knowledge among several computer systems. We describe the role of ontologies in supporting knowledge sharing activities in medicine Principles for the design of ontologies have been proposed, mainly in other domains: these principles include parsimony, clarity, representation of categories versus terms, and coherence. We analyze how and why these principles can or cannot be applied from case studies from medical systems. Regarding the fact that most of medical concepts are empirical, selected design decisions are discussed. An alternative representation choice consists in mapping principled general core ontologies and domain ontologies.